Priming of soil organic carbon decomposition induced by corn compared to soybean crops

The rate of soil organic carbon (CS) loss via microbial respiration (decomposition rate k, y−1), and the rate of stabilization of vegetation inputs (CV) into CS (humification rate h, y−1) are usually considered independent of CV. However, short-term laboratory studies suggest that the quality and quantity of CV controls k, which is often referred to as a priming effect. We investigated how the chemical composition of different residues, (corn and soybean) controls k and h under field conditions in a no-till ecosystem. Using CV-driven shifts in δ13C, we estimated changes in carbon (C) stocks, k and h of both the labile particulate organic matter fraction (CPOM) and the stabilized mineral associated organic matter fraction (CMAOM). After two years of high C inputs (corn: 4.4 Mg ha−1 y−1 aboveground and C:N = 78; soybean: 3.5 Mg ha−1 y−1, C:N = 17), we found no changes in CPOM and CMAOM stocks in the top 5-cm of soil or in deeper layers. However, CMAOM in corn had higher k (0.06 y−1) and C output fluxes (0.67 Mg ha−1 y−1) than in soybean (0.03 y−1 and 0.32 Mg ha−1 y−1), but similar rates and fluxes in CPOM in the top 5-cm of soil. In addition, while C inputs to CPOM were also similar for both crops, C inputs from CV to CMAOM were higher in corn (0.51 Mg ha−1 y−1) than in soybean (0.19 Mg ha−1 y−1). Overall, corn plots had higher k and C inputs into CMAOM and therefore higher C cycling in this fraction. Our data suggests that the type of crop residue strongly influences C cycling in the topsoil of no-till cropping systems by affecting both the stabilization and the decomposition of soil organic matter.